1. Field of Invention
This invention is in the field of Synthetic Aperture Radar image focusing with-out platform position updates from an inertial navigation (INS)/Ground Positioning Satellite (GPS) system.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) radar is used for ground mapping as well as target identification. The general principle behind SAR is to obtain high resolution images by coherently combining the amplitude and phase information of separate radar returns from a plurality of sequentially transmitted pulses from a relatively small antenna on a moving platform. The returns from the plurality of pulses transmitted during a SAR image, when coherently combined and processed, result in image quality comparable to a longer antenna, corresponding approximately to the synthetic “length” traveled by the antenna during the acquisition of the image.
High resolution SAR maps are obtained by coherently combining return signals reflected from transmitted pulses in the cross range direction from radar platform movement. Historically, formation of focused SAR images or maps required accurate information on platform position and velocity to shift and focus the received radar returns so as to have a useful, detailed combination of pulse returns from multiple pulses transmitted at different times from different radar positions. The process of aligning pulses in time and space for coherent integration is referred to as motion compensation, and is usually performed with the raw radar data, at the early stage of the image formation process.
The plurality of returns forming the image generated by the transmitted pulses along a known path of the platform make up an array length. During the array length, amplitude as well as phase information (in phase (I) and quadrature(Q) components) returned from reception of returns from each transmitted pulse, for each of many range bins, is preserved. The SAR image is formed and focused from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the array length.
One aspect of achieving coherent integration of pulses into one image is the need for an INS/GPS to indicate the spatial and time coordinates of each transmitted and received (or reflected) pulse. These time and space coordinates of radar returns need to be known to a relatively high accuracy, typically in fractions of a wavelength, to arrive at a clear, focused, unsmeared image. The INS/GPS is disadvantageous. An INS/GPS is an expensive, heavy and computationally burdensome item, comprising accelerometers as well as acceleration/velocity integrating means to ascertain moving platform location as a function of time. The INS/GPS presents a computationally intensive requirement of time stamping each individual radar return. This is followed by subsequent coherently alignment in time/space with the other SAR radar returns forming the image with respect to a central image feature.
Attempts at reducing the accuracy of an INS/GPS system to arrive at a focused SAR image have been previously discussed. Autofocus phase error estimates and corrections have been made where the INS/GPS information was too coarse, or inaccurate, precluding a well focused image from using platform motion information only. Such an autofocus improvement was described in U.S. Pat. No. 4,924,229 to P. H. Eichel, D. G. Ghiglia, C. V Jakowatz, Jr. titled Phase Correction System for Automatic Focussing of Synthetic Aperture Radar, May 8, 1990.
The need for a INS/GPS remained, along with its expense, and contribution to reduced SAR radar reliability and increased complexity.